Generally, a time adjustment for operations is made when a radar apparatus is newly installed on a ship or the like. This adjustment makes positions of radar images displayed on an indicator in a distance direction correspond with their actual distances with respect to the radar antenna.
Since, generally, a time duration from an instant at which a trigger pulse is supplied from a display unit to an antenna unit to another time instant at which a resultant echo rises consists of a time for a round trip of a radio signal to an object and an associated delay time, a timing adjustment is necessary. Causes of the delay time are signal delays due to cables between a display unit and an antenna unit, a delay in a transmission circuit and a delay in a receiving circuit, and the like.
Conventionally, in order to cope with the problem, a timing adjustment was made so that reception signal data can be sampled starting at a point corresponding to zero in distance from an antenna, when a radar is installed on a ship or the like.
In order to do the timing adjustment, as shown in FIG. 6, a time delay amount adjusting circuit "c" is provided between a transmission trigger pulse generating unit "a" for generating a transmission trigger pulse and a sampling circuit "b" for digitizing reception signals at a predetermined sampling period, and the transmission trigger pulse is supplied to the sampling circuit "b" through the delay amount adjusting circuit "c" as a start pulse. When a timing adjustment is to be made, search radio signals are emitted to an experimental object placed at a distance from the antenna and the time delay amount given to the transmission trigger pulse is manually adjusted at the delay amount adjustment circuit "c" so that image distortions are not generated.
In conventional radar apparatuses, it has been required to make timing adjustments so that reception signals can be sampled starting at a point corresponding to zero distance from an antenna, when a radar apparatus is newly installed on a ship, or when a cable is replaced with another cable having another length or when circuit components in transmitting or receiving units are changed. Thus, adjustments have been cumbersome.
Conventionally, a mono-multivibrator IC has been used to initiate to sample reception signals after a time elapses from a time instant at which a trigger signal is generated. Thus, the delay time has been adjusted by varying CR time constant.
Some signals produced by a conventional radar apparatus are shown in a form of timing chart shown in FIG. 11. Referring to FIG. 11, reception signals appear after a time "t.sub.0 " elapses from an instant at which a trigger signal is generated. One shot pulse is produced lasting a time "t.sub.0 ". A primary memory successively stores reception signals after the end edge of the one shot pulse. Reception signals stored in the primary memory are transferred to a secondary memory when sampling operations are not being made. There is shown in FIG. 12 an example for transferring data from the primary memory to the secondary memory. Write addresses of the secondary memory correspond to a pointing direction of the antenna, and, as shown in the figure, the first memory element of the primary memory corresponds to the center of the secondary memory, i.e., the position of the antenna. The data stored in the primary memory are successively transferred to corresponding memory elements of the secondary memory.
Such conventional apparatuses for controlling the start time to sample reception signals works effectively when a comparatively short one shot pulse is produced. But, when the CR time constant is to be made longer to lengthen the delay time, the period of one shot pulse will vary by a larger amount than the period of sampling pulses, and thus jitters are produced.
For example, with a surveillance radar used in a fish raising farm, there is sometimes used a long cable having a length of 1 km or 3 km. In such cases it is desired to produce a long-lasting and stable one shot pulse.
It is to be noted that in order to cope with the problem, digital circuits for counting clock signals to produce a desired delay signal may be used. In this case, however, an extremely high-speed operation is required. For example, with a range of 0.25 NM and a radius of 240 dots, the sampling frequency of clock pulses for each dot will be 77.7 MHz. It is difficult to construct a delay circuit operating at this speed with digital circuits at a low cost.